An Overview of the Recent HST Safings An Overview of the Recent HST Safings Ken Sembach TIPS Presentation September 20, 2007

20-Sep-2007Sembach/TIPS2 ZGSP Safe Mode Entry HST entered zero-gyro sun-point safe mode at ~7 PM EDT on August 31, 2007 Timeline of event 243/22:43:00 LOS 243/22:51:59 Enter orbit night 243/22:53:33 Two-Gyro transition to M2G mode 243/22:53:33 Type 2 vehicle slew (13 min 33 sec duration) Gyro Bad Disparity count exceeded limit of 1500 counts 243/23:01:54 Two-Gyro configuration failed (Zero-Gyro Drift Mode – AD commanded closed) 243/23:27:06 Enter orbit day - Sun Bright Object Protection failed Begin Zero-Gyro control 243/23:31 Expected AOS 243/23:39 Acquired telemetry 244/03:51 Engineering data dumped from recorder 244/05:52 OR# Power on Gyro 6

20-Sep-2007Sembach/TIPS3 Gyro #2 Failure HST was preemptively placed in two-gyro mode in August 2005 to conserve gyro lifetime Gyro #2 failure HST responded as expected Eventual failure of Gyro #2 was anticipated HST had reached approximate date of 50% probability of 4 working gyros HST had reached approximate date of 50% probability of 4 working gyros Most likely cause: flex lead failure Corrosion of thin electrical wire (“traditional” flex lead, no silver plating) Corrosion of thin electrical wire (“traditional” flex lead, no silver plating) Corrosion a function of current, diffusion rates, wire inhomgeneities Corrosion a function of current, diffusion rates, wire inhomgeneities Had improved fluid and fluid fill process (pressurized nitrogen vs. air) Had improved fluid and fluid fill process (pressurized nitrogen vs. air) Restart of Gyro #2 was deemed infeasible (failure permanent) Gyro #6 was powered on at ~2 AM EDT on 1-Sep-2007 Gyro #6 had been turned off early in its lifetime Gyro #6 shows some bias drift (noise) Gyro #6 bias is settling slowly (1-2 months expected)

20-Sep-2007Sembach/TIPS4 Gas-Bearing Gyroscope

20-Sep-2007Sembach/TIPS5 Gyro Status Full complement of gyros (6) was replaced during SM3A in December 1999 Gyro run times (31-Aug-2007) G hours- operational, powered G hours- failed, 31-Aug-2007, flex lead G hours- failed 29-Apr-2003, rotor restriction G hours- operational, turned off 28-Aug-2005 G hours- failed 28-Apr-2001, rotor restriction G hours- operational, powered All 6 gyros (3 rate sensing units - RSUs) are slated to be replaced during SM4

20-Sep-2007Sembach/TIPS6 Gyro Lifetime Estimates Predictions from Helen Wong (Aerospace Corp) as communicated to Art Whipple (HSTP) Chart below shows approximate gyro failure probabilities assuming a gyro failure in July 2007 (close to time of Gyro #2 failure) Jul-2007 May-2008 Jan-2009 Feb-2011 SM4

20-Sep-2007Sembach/TIPS7 Impact to Science Safing occurred near end of week (late Friday) ~25 orbits of science deferred (see Adler talk) Health and safety SMS loaded 1-Sep-2007 (Saturday) Science SMS loaded 2-Sep-2007 (Sunday) All WFPC2 Time critical WFPC2 orbits from previous week rescheduled Time critical WFPC2 orbits from previous week rescheduled No NICMOS orbits because of unrelated NICMOS safing event Congratulations to entire team here and at GSFC on speedy recovery!

20-Sep-2007Sembach/TIPS8 One-Gyro Preparations One-gyro mode is in an advanced stage of preparation, work begun in mid-2005 To be used only if necessary Target availability should be similar to two-gyro mode Target availability should be similar to two-gyro mode Jitter should be slightly larger than in two-gyro mode, but still very good Jitter should be slightly larger than in two-gyro mode, but still very good Initial PCS estimates indicate <10 mas persistent disturbance jitter (60-sec, RMS) Power on Gyro #4 if either Gyro #1 or Gyro #6 fails prior to SM4 Power on Gyro #4 if either Gyro #1 or Gyro #6 fails prior to SM4 Gyro #2 failure has not significantly altered one-gyro work schedule On-orbit test planned during week of 28-Jan-2008 External observations with WFPC2 and FGS External observations with WFPC2 and FGS PSF width and jitter impact (Sirianni: 11077) FGS astrometry (Nelan: 11078) Pointing and control maneuvers (Reinhart: 11345) Pointing and control maneuvers (Reinhart: 11345) Checkout similar to two-gyro mode checkout Checkout similar to two-gyro mode checkout

20-Sep-2007Sembach/TIPS9 NICMOS Safing NICMOS entered safe mode at ~11 PM EDT on 1-Sep-2007 NICMOS was in SAA/Operate mode as a result of previous HST safing No NICMOS activities at the time of event HST was not in SAA at time of safing HST power profile nominal at time of safing Expected PDU3 current drop seen in response safing Expected PDU3 current drop seen in response safing NICMOS telemetry and NED telemetry prior to safing appeared normal No out-of-family issues with ACS or WFPC2 at time of safing Error: A/D FIFO empty prior to reading all expected data Appropriate response is to safe instrument Buffer read every 0.5 seconds Many (tens of thousands) normal reads between HST safing event or two-gyro recovery and NICMOS safing => unrelated events Many (tens of thousands) normal reads between HST safing event or two-gyro recovery and NICMOS safing => unrelated events

20-Sep-2007Sembach/TIPS10 NICMOS Safing (continued) Cause: Single event upset (SEU) affecting the engineering data buffer, a CPU register, or a memory location “Verified” on VSTIF ops bench Recommended action was to recover from safe mode NICMOS was brought up to SAA/Operate mode Telemetry nominal Normal engineering data process functioned as expected Buffer box temperatures were raised to nominal values Had been running cold due to NICMOS inactivity Brief transition to Operate and back to SAA/Operate to refresh buffer box telemetry NICMOS science resumed with SMS loaded 9-Sep-2007